Coal comprises a mixture of hydrocarbons and carbohydrates, with small amounts of nitrogen, sulfur, water, and minerals. Coal burns in air with a yellow, smoky flame, leaving ash behind. The energy content of coal depends upon its type. The heat of combustion of brown coal or lignite, for example, is about twenty-five kJ/g, and the heat of combustion of bituminous coal and anthracite is about thirty-two kJ/g. When coal burns, it mainly produces water and carbon dioxide, however it also produces harmful sulfur dioxide, carbon monoxide, hydrocarbons, particulate matter and soot, and oxides of nitrogen (hereinafter “NOx”).
Coal is also the cheapest and most abundant fuel on the world. As a consequence, any technology that allows the use of coal in a cleaner way is necessarily very attractive. Clean coal technologies require, among other things, more reactive coal in order to reduce or eliminate particulate matter and soot, carbon monoxide, hydrocarbons and NOx's emissions. More reactive coal implies complete combustion of coal particles and improved access to reactants or adsorbants to coal surface.
One study that was conducted by Davis et al., uses advanced calculations demonstrating that only coal particle sized below eighteen microns, will burn completely inside a 900 MW tangentially fired boiler retrofitted with low NOx burners (Davis et al., “Evaluating the Effects of Low-NOx Retrofits on Caron in Ash Level”, Reaction Engineering International; Presented at the Mega Symposium: EPRI-DOE-EPA Combined Utility Air Pollutant Control Symposium in Atlanta, Ga., August, 1999). It is important to note, however, that currently commercial pulverized coal is typically ground to sixty micrometer average diameter. Further, commercial micronized coal has about fifteen microns average particle size, which means that a significant portion of the particles sizes are above the eighteen micron size, therefore contributing to the carbon in ash content. The Davis et al. study in view of the present invention is incorporated herein by reference.
Decreasing coal particle size implies increasing specific surface area, thereby increasing reactivity. Reducing particle size and obtaining a more reactive coal, opens many other applications, namely, as a feedstock for conventional but less polluting boilers; as a reburn fuel to reduce NOx emissions; as a feedstock of gasification and Oxycoal units; and as a feed in diesel and gas turbines. Further, coal cleaning processes are greatly enhanced by increasing specific surface area, facilitating the extraction of polluting minerals and solid compounds. Hereafter follows a description of these applications and the way they would benefit by using a micronized coal.
Boilers are closed vessels in which water or other fluids are heated. The heated or vaporized fluids exit the boiler for use in various processes or heating applications. In particular, utility boilers, which are typical drum-type boilers, are widely used in power plants, oil refineries, and petrochemical plants for steam generation to drive large turbines, producing electricity. In many instances, these boilers are coal-fired using coal at the burner to produce heated gases used to heat water, thereby generating steam.
Several decades ago, large utility boilers were fitted with pulverized-coal burners designed to fire pulverized coal using about fifteen percent to about twenty percent excess air. Under such conditions, the amount of unburned fuel normally was below two percent, although NOx levels generated by such burners reached levels that are now unacceptable according to current emission standards. In order to meet the current emission standards, low NOx burners have been developed and most commercial coal-fired boilers have been retrofitted with these low NOx burners. Low NOx burners operate to minimize NOx formation by introducing coal and its associated combustion air into a boiler such that initial combustion occurs in a manner that promotes rapid coal devolatilization in a fuel-rich (i.e., oxygen deficient) environment and introduces additional air to achieve a final fuel-lean (i.e., oxygen rich) environment to complete the combustion process. Using these low NOx burners reduces the NOx emissions up to about fifty to about sixty percent.
An example of a low NOx combustion system, such as a boiler with a low NOx burner, available from GE Power Systems is illustrated in FIG. 1. Such a system can include a reburn zone including reburn fuel injectors. The reburn zone is a technology that utilizes fuel and air staging to reduce the NOx emissions by integrating low NOx burners and over-fire air systems. Reburning is defined as reducing the coal and combustion air to the main burners and injecting a reburn fuel, such as coal, gas or oil, to create a fuel-rich secondary combustion zone above the main burner zone and final combustion air to create a fuel-lean burnout zone. The formation of NOx is inhibited in the main burner zone due to reduced combustion intensity, and NOx is destroyed in the fuel-rich secondary combustion zone by conversion to molecular nitrogen. A summary of GE Power System's technology is included in its publication entitled “Reburn Systems” having reference number GEA-13207, which is incorporated herein by reference.
However, the use of low NOx burners increases the carbon content, or unburned coal, in the boiler ash. FIG. 2 depicts measurements taken from a utility boiler firing a ten percent ash coal. The results show the increase of carbon in ash content after retrofitting the boiler with low NOx burners. Although the increase of the amount of unburned carbon can also be boiler and coal dependent, Table 1 shows a common trend toward the increase of carbon in ash data from several boilers fitted with low NOx burners.
TABLE 1Select Boilers for Which Detailed Carbon in Ash AnalysesHave Been PerformedTypicalTypicalMeasuredMeasuredFiringLow NOxNOxCarbon inConfigurationMWeSystemEmissionsAsh LevelOpposed wall500FW CFSF313 ppm5%firedburners withAOFAOpposed wall500FW CFSF310 ppm8%firedburners withoutOFASingle wall160DBRiley CCVII245 ppm22-27%firedburners and OFATangentially900ABB LNCFS275 ppm 8-12%FiredLevel III
The disposal of boiler ash with increased carbon content is becoming a pressing issue within the power utilities markets and will continue to be more so in the future, as the cost of coal and other fuels continue to rise.
One method of utilizing coal as a fuel for utility burners is to create a slurry or dispersion of the coal. For example, the coal is pulverized and mixed with an amount of water in order to form a dispersion or slurry of coal in water at a low enough viscosity so as to enable transportation of the fuel via pipeline or the like. However, because the pulverized or micronized coal is only available at the particle sizes described above, the pulverized coal does not completely burn, and therefore the coal in water slurry does not solve the issues of high carbon content in boiler ash as described above.
Gas turbines can also utilize coal as fuel. A gas turbine is a rotary machine, similar in principle to a steam turbine. It consists of three main components—a compressor, a combustion chamber and a turbine. Air, after being compressed into the compressor, is heated either by directly burning fuel in it or by burning fuel externally in a heat exchanger. The heated air, with or without combustion products, is expanded in a turbine resulting in work output, a substantial part of which is used to drive the compressor. The excess is available as useful work output. In one example, a gas turbine has an upstream air compressor mechanically coupled to a downstream turbine, with a combustion chamber positioned in between. Energy is released when compressed air is mixed with fuel, such as coal, which is then ignited in the combustion chamber. The resulting gases are directed over the turbine's blades, spinning the turbine, and mechanically powering the compressor. Finally, the gases can be passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure. Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, electrical generators, and even tanks.
However, commercially available coal-in-water slurries are not conducive to gas turbine applications. When the pulverized or micronized coal is combined with the compressed air and burned, the presence of unburned coal particles can damage the turbine blades, resulting in a less efficient process, and significant expense in replacing the turbine blades.
In diesel engines, a diesel engine relies upon compression ignition to burn its fuel. If air is compressed to a high degree, its temperature will increase to a point where fuel will burn upon contact. Following intake, the cylinder is sealed and the air charge is highly compressed to heat it to the temperature required for ignition. As the piston approaches top dead centre (TDC), fuel oil is injected into the cylinder at high pressure, causing the fuel charge to be nebulized. Owing to the high air temperature in the cylinder, ignition instantly occurs, causing a rapid and considerable increase in cylinder temperature and pressure. The piston is driven downward with great force, pushing on the connecting rod and turning the crankshaft. If commercially available coal-in-water slurries are used as the fuel, the presence of unburned coal particles after combustion of these fuels can cause damage to the cylinders, such as damaging the tolerances between the piston and the cylinder. This in turn may cause damage or failure to the seal of the cylinder, resulting in a lack of pressure to increase the temperature to ignite the fuel, for example.
Coal can also be used as a combustion fuel for a gasification process. Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen. The resulting gas mixture is known as synthesis gas or syngas, which can in turn be used as a fuel. The syngas product can be burned directly as a fuel in internal combustion engine, processed into high-purity hydrogen, ammonia, methanol, and other chemicals, or converted via the Fischer-Tropsch process into synthetic fuel. However, commercially available coal-in-water slurries produce a lower quality or contaminated syngas because of the presence of unburned coal particles, as well as clogging of the particulates in the input stream. One example of a gasification process is the Texaco Gasification Process entitled “EPA: Site Technology Capsule—Texaco Gasification Porcess” having reference EPA 540/R-94/514a of April 1995, which is incorporated herein by reference.
There remains a need for a “green” coal to be used as in a coal-in-water slurry as a fuel for multiple applications including low NOx burners, gasification processes, gas turbine applications, diesel engine applications, and the like. Such “green” coal should completely burn, leaving no coal particulates in the downstream ash, products, and/or byproducts.